1. Field of the Invention
The present invention relates to improved water soluble fertilizer compositions. More particularly, it relates to solid fertilizer compositions having sufficiently high acidifying effect (i.e. alkalinity reducing effect) and water solubility to provide stable, precipitate free stock and feed solutions, independent of the phosphorus content thereof.
2. Description of Related Art
In greenhouses, nurseries and other intensive horticulture environments, best plant growth results are achieved when macro and micro nutrients are carefully delivered to the growing plants. Many plant growers choose to utilize compound high analysis water soluble fertilizers for accomplishing this result. Typically, such high analysis fertilizers are marketed as solids which may be dissolved by the user to prepare concentrated stock solutions which are subsequently diluted into irrigation water employing proportioners or injection devices, thereby forming so called "feed solutions".
Generally, it is important to formulate these high analysis fertilizer compositions so that they dissolve quickly and completely with no precipitation. Furthermore, these fertilizer compositions must provide good long-term stability in stock solutions. For example, precipitates in the stock solutions can cause clogging of the proportioners and irrigation lines. These functional requirements for water soluble fertilizer compositions of this variety have presented ongoing problems for producers and developers of such products and these problems have not been fully solved by previously available fertilizer compositions.
For example, when previously conventional fertilizer compositions have been employed for preparing fertilizer irrigation solutions from hard water and/or water having high alkalinity, problems have been encountered with respect to precipitation. Irrigation water having a relatively high alkalinity normally contains a high content of (hydrogen)carbonates and the presence of such materials very often results in precipitation of secondary macro nutrients and micro nutrients (trace elements) from the fertilizer solution. Specifically, calcium is an important secondary macro nutrient which is required in many plant fertilizer compositions. However, a relatively high pH will cause calcium and non-chelated micro nutrients to precipitate in the form of (hydrogen)carbonates, phosphates, sulfates and/or hydroxides.
Heretofore, various solutions have been proposed to decrease the alkalinity and consequently to reduce the precipitation of nutrients. For example, in U.S. Pat. No. 5,830,255 and 5,174,806, phosphorus containing acids were disclosed for use as acidifiers in the fertilizer compositions.
U.S. Pat. No. 5,830,255 describes a liquid fertilizer composition including phosphorous acid (H.sub.3 PO.sub.3) as the primary macro nutrient. The composition may further include other nutrients and, additionally, contains citric acid. It is taught that this composition improves the uptake by plants of phophites (PO.sub.3.sup.3-) which are the salts of phosphorous acid. Phosphites are said to be taken up by the foliage of some plants more readily than phosphates and are, therefore, preferred for these plants.
The problem underlying the invention described in U.S. Pat. No. 5,830,255 is the precipitation of phosphates, which are used as primary macro nutrients, which precipitation occurs in conventional fertilizer compositions comprising phosphorous acid and other nutrients. In an attempt to solve this problem, citric acid is included in the fertilizer composition which, when diluted with water having a pH of about 6.5-8.5, results in a pH of 5.0-7.0. The effect of using citric acid is, therefore, lowering of the pH in order to prevent the precipitation of the phosphites.
U.S. Pat. No. 5,174,806 describes a method for preparing a fertilizer composition including phosphoric acid (H.sub.3 PO.sub.4) as the primary macro nutrient, other nutrients and additionally citric acid and urea. This fertilizer composition is said to be neutral and to prevent the evolution of heat of neutralization between the phosphoric acid and other nutrients such as potassium hydroxide due to the presence of citric acid and urea in the formulation.
However, it should be noted that the use of phosphorus containing acids, such as phosphorous acid and phosphoric acid as proposed in the above-cited prior art, has several disadvantages.
One disadvantage is that phosphorus containing acids are liquids. Therefore, a grower wishing to fertilize with, for example, both calcium and a phosphorus containing acid will need to inject these two elements separately This makes other non-phosphorus containing acids such as nitric acid, sulphuric acid, formic acid, acetic acid and the like, unuseable in the formulation. Another disadvantage is that phosphorus containing acids are hazardous in handling and application. Thirdly, liquid acids cannot be blended properly with solid fertilizer nutrients in powder form to produce a fertilizer composition. Therefore, manufacturing of such compositions, especially with phosphorus containing acids, is troublesome. Additionally, chelates (e.g., being used as micro nutrients) are unstable in combination with liquid acids.
In view of the foregoing disadvantages, other solutions to the problem of decreasing the alkalinity and, consequently, reducing the precipitation of nutrients were sought and one proposal was to use of certain phosphates in the formulation.
For example, International Publication WO 92/013813 describes the application of urea phosphate in water soluble solid fertilizer compositions. This publication discloses that the use of mono-ammonium phosphate (MAP) in fertilizer mixtures with calcium nitrate, magnesium nitrate, ferrous sulphate, manganese sulphate, copper sulphate, zinc sulphate and the like results in solutions with precipitates. However, the publication teaches that when urea phosphate (UP) is employed as the principal phosphorus source in a fertilizer mixture with the same secondary and micro nutrients, the use of UP will permit calcium nitrate, magnesium nitrate and/or metal sulfates to be present in clear, concentrated stock solutions. This is a result that MAP, as the principal phosphorus source, is incapable of providing.
Also, in view of the acidifying effect of UP on irrigation water, alkalinity can be reduced considerably which results in a significant increase in solubility for calcium and non-chelated micro nutrients, especially in case of hard water and/or water of high alkalinity, and the reduction in the concentration of hydrogen carbonate avoids precipitation in irrigation water of high alkalinity. Thus, depending on the alkalinity, water hardness and composition of the fertilizer, this publication teaches that a certain amount of UP in the fertilizer composition is needed.
A disadvantage to the solubility problem solution provided by International Publication WO 92/013813 is that more conventional phosphorus sources, such as mono-ammonium phosphate, mono-potassium phosphate and di-ammonium phosphate, cannot be used, for example, with calcium salts to produce a precipitate free solution. To the contrary, urea phosphate must be used as the principal phosphorus source instead of these more common materials.
Another disadvantage is that for some plants or in some stages of growth, a high phosphorus level in the irrigation water is not desirable. Problems occur when a certain amount of UP is needed for hydrogen carbonate neutralization, i.e. acidifying, effect and, simultaneously, a relatively high phosphorus level is undesirable from a plant growth perspective.
Additionally, other solutions have been proposed heretofore for reducing the precipitation of nutrients in fertilizer compositions.
For example, the application of low-molecular-weight organic acids like oxalic acid, citric acid and the like to the soil to increase the absorption of phosphates, used as primary macro nutrients, by plants is known (Biology and Fertility Soils, Vol. 18, No. 4, 1994, pp. 311-319). The organic acids increase the availability of phosphorus in soils mainly through both decreased adsorption of phosphates to the soil and increased solubility of the phosphorus compounds. These organic acids are applied separately from the phosphates. Accordingly, the fertilizer composition used does not comprise any organic acids.
Another widely postulated proposal for solving the solubility problem is the use of chelated micro nutrients (trace elements) in fertilizer compositions in order to keep the micro nutrients in solution in both stock solution and irrigation water, containing phosphate salts as well. The use of chelated micro nutrients is necessary since when non-chelated micro nutrients (such as simple nitrates or sulfates) are used with conventional phosphorous sources, the micro nutrients tend to precipitate. A disadvantage of such use is that chelated micro nutrients increase the cost of the fertilizer compositions considerably.
Thirdly, the application of several types of acids which have a complex-forming (i.e. chelating) effect are well known in fertilizer compositions to stabilize metals (such as micro nutrients) by avoiding precipitation of the metals. Examples of such acids include ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA) and the like. A disadvantage is that these chelating acids are not soluble enough (less than 10 g/l at 25.degree. C.) to be used in solid water soluble fertilizers. Moreover, these acids have a low acidifying effect.